Streambed periphyton communities are complex associations of autotrophic and heterotrophic microorganisms. Periphyton contributes to primary production and nutrient cycling and serves as food and habitat for stream fauna. Periphyton biomass growth is often limited by the availability of nitrogen and phosphorus in streams, although bottom-up effects of nutrients interact with light levels, temperature, and current velocity. Periphyton biomass can be strongly depleted by stream herbivores (grazers), and top-down effects are regulated by factors such as predators, temperature, and current velocity. While studies have often manipulated nutrients and grazers simultaneously, few studies have examined the context-dependency of these interactions. In this study, we sought to understand seasonal shifts in the importance of top-down and bottom-up factors in a stream dominated by aquatic insect grazers. We used nutrient diffusing substrates to add nutrients and an electric field to exclude insects in three sequential experiments from August – October 2017. Our response variables included algal biomass, organic matter, and algal community composition (% green algae, % cyanobacteria, and % diatoms). We analyzed the data using ANOVA models, with a random block (replicate plots), whole plot factor (electricity), and subplot factor (nutrients).
Results/Conclusions
Grazers significantly reduced periphyton biomass in the first experiment but not the subsequent experiments, likely because a dominant mayfly grazer in the stream was most abundant during that time. Grazing significantly reduced diatoms in the first experiment but reduced green algae in the second and third experiments, reflecting the changing feeding roles of the insect communities. Phosphorus was the primary limiting nutrient for algal biomass but did not influence organic matter across all experiments. Effects of nutrient additions on algal biomass and community composition decreased over time.
Grazers did not have a dominant influence on algal biomass for two likely reasons. First, we completed short, 16-day experiments whereby algal communities never surpassed an early successional stage. Biomass was tightly bound to growth discs and largely unavailable to grazers. Second, many estimates of grazing rates are obtained from laboratory experiments, which tend to produce higher estimates than field experiments. Nutrients had a strong effect as has been found in other studies, but the effect likely decreased over time because temperature became more limiting to periphyton growth than nutrients. This study contributes to community ecology by examining the temporally variable responses of algal biomass and community composition to top-down and bottom-up factors.